Use of a Polyolefin Composition for Pipes and Fittings With Increased Resistance to Chlorine Dioxide

ABSTRACT

The present invention relates to the use of a polyolefin composition for water pipes or fittings with increased resistance to degradation caused by chlorine dioxide-containing water, particularly to the use of a combination of particular types of antioxidants for increasing the resistance of a polyolefin composition against degradation caused by contact with chlorine dioxide-containing water.

The present invention relates to the use of a polyolefin composition forwater pipes or fittings with increased resistance to degradation causedby chlorine dioxide-containing water, particularly to the use of acombination of particular types of antioxidants for increasing theresistance of a polyolefin composition against degradation caused bycontact with chlorine dioxide-containing water.

Recent progresses in the manufacturing and processing of polymers haveled to the application of plastics in virtually every aspect of modernday life. However, polymeric compounds are prone to aging under theeffects of oxidants, light and heat. This results in a loss of lifetimesuch as loss of strength, stiffness and flexibility, discoloration andscratching as well as loss of gloss.

It is well-known in the art that antioxidants and light stabilizers canprevent or at least reduce these effects. Several types of additives areadded to polymers to protect them during processing and to achieve thedesired end-use properties. Additives are generally divided instabilizers and modifiers. Stabilizers, like antioxidants, traditionallyand currently used comprise sterically hindered phenolics, aromaticamines, hindered amine stabilizers, organo-phosphites/phosphonites andthioethers. However, appropriate combinations of stabilizers have to becarefully selected, depending on the desired final properties, thepolymeric article should have.

Besides many other applications, polyolefins are used for thepreparation of pipes or fittings for drinking water distributionsystems. In the following disclosure all characteristics and effectssupplied to pipes are also supplied to fittings, even if not mentionedspecifically. To ensure that the drinking water intended for humanconsumption is of good quality, disinfection is often used. Disinfectionmeans the removal, deactivation or killing of pathogenic microorganisms.

It is known that chlorine is used as disinfectant in water treatment toprevent spread of infectious diseases. The chlorine source for theresulting chlorinated water is chlorine gas or hypochlorite. There is achemical equilibrium of chlorine, hypochlorite and chloride in waterwhich is known by the skilled person.

One of the disadvantages of chlorinated water is the presence ofchlorine-containing reaction products of chlorine with organic materialin the drinking water. Some of said reaction products are discussed toprovide health risks.

It is also known that most materials, including many polymers such aspolyolefins, age in chlorinated water. Results from pressure testing inlaboratories and experience from the field have shown that highconcentration of chlorine in water can cause early brittle fracture inpolyolefin pipes.

An effective alternative disinfectant for drinking water systems ischlorine dioxide, ClO₂. Chlorine dioxide is a stronger oxidizing agentthan chlorine and does not form halogenated by-products. It does nothydrolyze in water but remains in solution as a dissolved gas.

Nevertheless, also chlorine dioxide-containing water is in permanentcontact with the pipe material. Due to the permanent contact to theinner pipe surface, deterioration of the polyolefin composition iscaused.

It has been found that antioxidants used in polyolefin compositions forpipes known to provide good resistance to chlorinated water do notnecessarily provide satisfactory resistance against chlorinedioxide-containing water. Thus, there is still a need for a moreefficient antioxidant which provides a better protection againstClO₂-containing water to a polyolefin composition, and thus allows for alonger lifetime of e.g. a pipe, made of a polyolefin compositioncontaining such an antioxidant.

A further important issue as regards the presence of antioxidants inpolyolefin compositions is the aim to avoid contamination of mediatransported e.g. in a pipe made of such a polyolefin composition. Thisis particularly important in case of a pipe transporting drinking water.Generally speaking, it is preferred to use as low concentrations ofantioxidant as possible in order to lower the amount of antioxidantwhich may possibly be extracted by the water transported in the pipe.Further in this context, it is desirable that the antioxidant used has alow tendency to extraction by the water transported in the pipe.

Hence, there is still a need for improved polyolefin compositionssuitable for water pipe applications, particularly for polyolefincompositions having an increased lifetime in contact to chlorinedioxide-containing water.

Thus, it is an object of the present invention to provide a polyolefincomposition for pipes having an increased lifetime in permanent contactwith chlorine dioxide-containing water.

The present invention is based on the finding that the object of theinvention can be achieved, if the polyolefin composition comprises aspecific combination of several types of antioxidants.

Therefore, the present invention relates to the use of a combination oftwo antioxidants (B) and (C) in a polyolefin composition for increasingthe lifetime of a pipe or a fitting made of said polyolefin compositionwhich pipe or fitting is in permanent contact with chlorinedioxide-containing water, wherein said antioxidants are the following:

-   -   antioxidant (B) with general formula R¹—P(OAr)₂, wherein OAr is        according to formula (I):

-   -   wherein        -   R¹ is a non-substituted or substituted aliphatic or aromatic            hydrocarbyl radical which may comprise heteroatoms,        -   R², R³, R⁴, R⁵ and R⁶ independently are a hydrogen atom or            non-substituted or substituted aliphatic or aromatic            hydrocarbyl radicals which may comprise heteroatoms,    -   and    -   antioxidant (C) according to formula (II):

-   -   wherein        -   R⁷, R⁸ and R⁹ independently are non-substituted or            substituted aliphatic or aromatic hydrocarbyl radicals which            may comprise OH-groups,        -   X¹, X², and X³ independently are H or OH, with the proviso            that at least one of X¹, X² and X³ is OH,        -   the entire molecule does not comprise an ester group.

R¹ preferably is an aliphatic or aromatic hydrocarbyl radical, whereinthe heteroatoms present are selected from oxygen, nitrogen and phosphoratoms.

In case of at least one phosphor atom it is particularly preferred thatR¹ is directly attached to the phosphor atom of formula R¹—P(OAr)₂ via acarbon atom of an aromatic hydrocarbyl radical which itself is directlyattached to a further phosphor atom of a further group P(OAr)₂ via adifferent carbon atom of said aromatic hydrocarbyl radical, i.e. R¹ isan aromatic hydrocarbyl radical bridging two P(OAr)₂-groups.

Preferably, the amount of carbon atoms in said bridging aromatichydrocarbyl radical is between 6 and 25, more preferably between 6 and20.

In an alternative particularly preferred embodiment R¹ is an aliphaticor aromatic hydrocarbyl radical wherein the only heteroatoms present areoxygen atoms, one of which connects R¹ with the phosphor atom of theP(OAr)₂-group. Still more preferably, said oxygen atom is the onlyheteroatom present.

In the above-mentioned alternative preferred embodiment it is furtherpreferred that R¹ comprises between 1 and 20 carbon atoms, morepreferably between 2 and 16 carbon atoms.

In said alternative preferred embodiment it is further preferred thatapart from said connecting oxygen atom R¹ does not comprise a furtherheteroatom and said oxygen atom connects an aliphatic hydrocarbylradical with the P(OAr)₂-group, preferably an unbranched alkyl chain.

In said alternative preferred embodiment it is further preferred thatapart from said connecting oxygen atom R¹ does not comprise a furtherheteroatom and said oxygen atom connects an aromatic hydrocarbyl radicalwith the P(OAr)₂-group, still more preferably R¹ is OAr as definedherein.

R², R³, R⁴, R⁵ and R⁶ preferably comprise between one carbon atom and 20carbon atoms each, more preferably not more than 10 carbon atoms each.

Preferably, R², R³, R⁴, R⁵ and R⁶ do not comprise any heteroatom.

Preferably, R², R³, R⁴, R⁵ and R⁶ are a hydrogen atom or aliphatichydrocarbyl radicals, more preferably alkyl radicals. Even morepreferred is that at least one of R², R⁴ and R⁶ is an alkyl radical,more preferably with one to four carbon atoms, more preferably a methylgroup or a tert-butyl group. Still more preferably, R³ and R⁵ arehydrogen atoms.

In a particular preferred embodiment at least two of R², R⁴ and R⁶ are atert-butyl group.

It is further preferred that R², R³, R⁴, R⁵ and R⁶ is the same in theP(OAr)₂-group and in the preferred embodiment wherein R¹ is OAr asdefined herein.

A particular preferred antioxidant (B) is tris (2,4-di-t-butylphenyl)phosphite (Irgafos 168).

In antioxidant (C) according to formula (II) residues R⁷, R⁸ and R⁹independently are non-substituted or substituted aliphatic or aromatichydrocarbyl radicals which may comprise OH-groups. This means that apartfrom OH-groups no further heteroatoms are present in R⁷, R⁸ and R⁹, sothat phenolic stabilizer (C) is e.g. free of ester groups, amide groupsand groups containing phosphorus.

Preferably, R⁷, R⁸ and R⁹ which independently are non-substituted orsubstituted aliphatic or aromatic, more preferably aliphatic,hydrocarbyl radicals which may comprise OH-groups, have from 2 to 200carbon atoms.

Preferably, R⁷ and R⁸ independently have from 2 to 20 carbon atoms, morepreferably from 3 to 10 carbon atoms.

Furthermore, it is preferred that R⁷ and/or R⁸, more preferably R⁷ andR⁸, are aliphatic hydrocarbyl groups, more preferably alkyl radicals,still more preferably with at least 3 carbon atoms which have a branchat the carbon atom connected to the aromatic ring, and most preferablyR⁷ and/or R⁸, more preferably R⁷ and R⁸, are tert-butyl groups.

Preferably, R⁹ has from 20 to 100 carbon atoms, more preferably has from30 to 70 carbon atoms.

Furthermore, it is preferred that R⁹ includes one or more phenylresidues.

Still further, it is preferred that R⁹ includes one or morehydroxyphenyl residues.

Preferably, in antioxidant (C) of formula (II) X¹ is OH, and mostpreferably X¹ is OH and X² and X³ are H.

It is particularly preferred that antioxidant (C) is1,3,5-Tri-methyl-2,4,6-tris-(3,5-di-tert. butyl-4-hydroxyphenyl) benzene(Irganox 1330).

Antioxidant (B) is preferably contained in the composition in an amountof 5000 ppm or less, more preferably 2000 ppm or less, still morepreferably 1200 ppm or less, based on the total composition.

The amount of antioxidant (C) in the polyolefin composition ispreferably 5000 ppm or less, more preferably 4450 ppm or less, morepreferably 3600 ppm or less, still more preferably 2500 ppm or less andparticularly preferred is 1300 ppm or less, based on the totalcomposition.

Usually, the composition will contain any of the antioxidants (B) and(C) independently in an amount of at least 50 ppm.

Preferably, the sum of concentration of antioxidants (B) and (C) isbetween 1500 and 6000 ppm, more preferably between 1800 and 5500 ppm.

The term “base resin” denotes the entirety of polymeric components inthe polyolefin composition according to the invention, usually making upat least 90 wt % of the total composition.

The favourable effect of the antioxidants according to the presentinvention is not dependent on the type of polyolefin base resin used.The base resin may therefore be any polyolefin or polyolefincomposition.

However, it is preferred that the base resin (A) comprises an ethylenehomo- or copolymer or a propylene homo- or copolymer. Preferably, thecomonomer is selected from ethylene and alpha-olefins with 4 to 8 carbonatoms. Still more preferably ethylene or an alpha-olefin selected from1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene is used.

The amount of comonomer in the base resin (A) is preferably between 0.1mol % and 7.0 mol %.

It is particularly preferred that the base resin (A) comprises anethylene homo- or copolymer, more preferably that the base resin (A)consists of an ethylene homo- or copolymer.

In one embodiment of the invention the base resin comprises two or morepolyolefin, more preferably polyethylene, fractions with differentweight average molecular weight. Such resins usually are denoted asmultimodal resins.

Polyolefin, in particular polyethylene, compositions comprisingmultimodal resins are frequently used e.g. for the production of pipesor fittings due to their favourable physical and chemical properties ase.g. mechanical strength, corrosion resistance and long-term stability.Such compositions are described e.g. in EP 0 739 937 and WO 02102891.The term molecular weight used herein generally denotes the weightaverage molecular weight M_(w).

As mentioned, usually a polyethylene composition comprising at least twopolyolefin fractions, which have been produced under differentpolymerisation conditions resulting in different weight averagemolecular weights for the fractions, is referred to as “multimodal”. Theprefix “multi” relates to the number of different polymer fractions thecomposition is consisting of. Thus, for example, a compositionconsisting of two fractions only is called “bimodal”.

The form of the molecular weight distribution curve, i.e. the appearanceof the graph of the polymer weight fraction as function of its molecularweight, of such a multimodal polyethylene will show two or more maximaor at least be distinctly broadened in comparison with the curves forthe individual fractions.

For example, if a polymer is produced in a sequential multistageprocess, utilising reactors coupled in series and using differentconditions in each reactor, the polymer fractions produced in thedifferent reactors will each have their own molecular weightdistribution and weight average molecular weight. When the molecularweight distribution curve of such a polymer is recorded, the individualcurves from these fractions are superimposed into the molecular weightdistribution curve for the total resulting polymer product, usuallyyielding a curve with two or more distinct maxima.

In a preferred embodiment wherein the base resin consists of twopolyethylene fractions, the fraction having a lower weight averagemolecular weight is denoted fraction (A), the other is denoted fraction(B).

Fraction (A) preferably is an ethylene homopolymer.

Fraction (B) preferably is an ethylene copolymer, and preferablycomprises at least 0.1 mol % of at least one alpha-olefin comonomer. Theamount of comonomer is preferably at most 14 mol %.

In a preferred embodiment wherein the polyolefin composition is apolyethylene composition, the base resin of the polyethylene compositionpreferably comprises at least 0.1 mol %, more preferably at least 0.3mol %, and still more preferably at least 0.7 mol % of at least onealpha-olefin comonomer. The amount of comonomer is preferably at most7.0 mol %, more preferably at most 6.0 mol %, and still more preferablyat most 5.0 mol %.

As an alpha-olefin comonomer, preferably an alpha-olefin having from 4to 8 carbon atoms is used. Still more preferably an alpha-olefinselected from 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene isused.

The polyolefin base resin preferably has an MFR₅ (190° C., 5 kg) of from0.1 to 2.0 g/10 min, more preferably from 0.2 to 1.5 g/10min, and mostpreferably from 0.5 to 1.0 g/10 min.

The density of the base resin preferably is from 930 to 960 kg/m³, morepreferably is from 935 to 958 kg/m³, and most preferably is from 936 to955 kg/m³.

In addition to the base resin and the antioxidants, usual additives forutilization with polyolefins, such as pigments (for example carbonblack), stabilizers, antiacids and/or anti-UVs, antistatic agents andutilization agents (such as processing aid agents) may be present in thepolyolefin composition.

The amount of such additives usually is 10 wt % or below.

Carbon black is a generally used pigment, which also acts as anUV-screener. Typically carbon black is used in a final amount of from0.5 to 5% by weight, preferably from 1.5 to 3.0% by weight. Preferablythe carbon black is added as a masterbatch, i.e. Carbon black masterbatch (CBMB) where it is premixed with a polymer, preferably highdensity polyethylene (HDPE), in a specific amount as e.g. shown in theExamples. Suitable carbon black masterbatches are, among others, HD4394,sold by Cabot Corporation, and PPM1805 by Poly Plast Muller.

The polymerisation catalysts for the production of the base resininclude coordination catalysts of a transition metal, such asZiegler-Natta (ZN), metallocenes, non-metallocenes, Cr-catalysts etc.The catalyst may be supported, e.g. with conventional supports includingsilica, Al-containing supports and magnesium dichloride based supports.Preferably the catalyst is a ZN catalyst, more preferably the catalystis a non-silica supported ZN catalyst, and most preferably a MgCl₂-basedZN catalyst.

The Ziegler-Natta catalyst further preferably comprises a group 4 (groupnumbering according to new IUPAC system) metal compound, preferablytitanium, magnesium dichloride and aluminium.

The catalyst may be commercially available or be produced in accordanceor analogously to the literature. For the preparation of the preferablecatalyst usable in the invention, reference is made to WO 2004055068 andWO 2004055069 of Borealis and EP 0 810 235. The content of thesedocuments in its entirety is incorporated herein by reference, inparticular concerning the general and all preferred embodiments of thecatalysts described therein as well as the methods for the production ofthe catalysts. Particularly preferred Ziegler-Natta catalysts aredescribed in EP 0 810 235.

The composition preferably is produced in a process comprising acompounding step, wherein the base resin which is typically obtained asa base resin powder from the reactor, together with the antioxidants andoptionally other additives is extruded in an extruder to yield thecomposition according to the present invention.

Of course, when using the inventive composition, further compoundsselected from conventional additives, fillers, minerals and lubricantsmay be added for improving processability and surface characteristicsthereof.

The composition of the present invention is preferably used in pipes orfittings—black as well as natural (i.e. non-colored) or coloredpipes/fittings. Preferably, such a pipe or fitting is used in a drinkingwater supply system. It is furthermore preferred that the pipe is a coldwater pipe, i.e. that it is designed for the transport of cold water.

Hence, the present invention is also directed to a pipe or fittingcomprising the inventive polyolefin composition mentioned aboveincluding all the preferred embodiments. Such pipes/fittings show animproved resistance against chlorine dioxide-containing water.

The pipes or fittings are preferably produced by extrusion of thepolyolefin composition.

The present invention is therefore also directed to the use of apolyolefin composition according to the invention, including all of thepreferred embodiments, for the production of a pipe or fitting.

The present invention is also directed to the use of said inventivepipes or fittings for the transport of chlorine dioxide-containingwater.

As mentioned above, the present invention is directed to the use of acombination of antioxidants (B) and (C) as defined above, including allof the preferred embodiments, in a polyolefin composition for increasingthe lifetime of a pipe or fitting made of said polyolefin compositionwhich pipe/fitting is in permanent contact with chlorinedioxide-containing water.

Preferably, said increased lifetime of the pipe or fitting is shown in ahoop stress test according to ASTM F 2263-03 compared to a pipe made ofa corresponding polyolefin composition without any antioxidant,preferably compared to a pipe made of a corresponding polyolefincomposition comprising antioxidants or a combination of antioxidantsother than the combination of antioxidants (B) and (C) in an equalconcentration.

A “corresponding polyolefin composition” denotes a polyolefincomposition which comprises the same base resin and the same additivesin the same concentration as the inventive polyolefin compositionbesides the combination of antioxidants (B) and (C).

The “equal concentration” of antioxidants addresses the sum ofconcentration of antioxidants (B) and (C) and the sum of concentrationof antioxidants in corresponding polyolefin compositions.

Preferably, said increased lifetime of the pipe or fitting is shown by afailure time of at least 2500 hours in a hoop stress test according toASTM F 2263-03.

EXAMPLES 1. Definitions and Measurement Methods

a) Density

Density is measured according to ISO 1183-1:2004 on compression mouldedspecimen prepared according to EN ISO 1872-2 (February 2007) and isgiven in kg/m′.

b) Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the flowability, andhence the processability, of the polymer. The higher the melt flow rate,the lower the viscosity of the polymer. The MFR is determined at 190° C.for polyethylene and determined at a loading of 5.00 kg (MFR₅).

c) Measurement of Lifetime of Pipes in Contact with ClO₂

No standard exists yet for evaluating the resistance of pipes comprisinga polyethylene composition to ClO₂-containing water. However, there is astandard for measuring the resistance to chlorinated water: ASTMF2263-03, “Standard test method for evaluating the oxidative resistanceof Polyethylene (PE) pipe to chlorinated water”. The lifetime of thepipes is tested accordingly with equipment according to ASTM F2263-03.However, ClO₂ is applied instead of chlorine.

A circulation loop is used for water which contains ClO₂. Theconcentration of ClO₂ in the water is 1.0±0.1 ppm. The pH of the wateris 6.8±0.2. The temperature of the water is 90±1 ° C. The hoop stressapplied to the pipe is about 1.7 MPa. The oxidation reduction potential(ORP) is 740 mV and is measured frequently. The flow volume is 23 l/h ata flow velocity of about 0.13 ms and a fluid pressure of 6.5 bar. Thefree pipe length is 250 mm, the outer diameter of the pipe is 12 mm andthe thickness of the wall is 2 mm. The condition time is 1 hour. In thetests two pipes of each material are tested in series. Each pipe istested until failure. The average of the two lifetime values iscalculated.

The circulation loop used for ClO₂ testing is made from inert materials(e.g. titanium, PVDF (Polyvinylidene difluoride), PTFE(Polytetrafluoro-ethylene) to avoid contamination of the test fluid. Thefittings are of PVDF. The test fluid is continuously purified in threesteps to avoid any contamination: 1. active carbon filter, 2. particlefilter, 3. reverse osmosis. The internal environment is theabove-mentioned solution of ClO₂ in water, the external environment isair.

The ClO₂ is generated directly at the site using a commercialClO₂-generator from Prominent following the equation:

5NaClO₂+4HCl→4ClO₂+2H₂O+5NaCl

The mechanism for feeding the stock solutions (NaClO₂ and HCl) to theprocess is monitored to maintain a consistent ratio of chemicals.

All tests were carried out at Bodycote Polymer AB, Nyköping, Sweden.

d) Content of Antioxidant

Sample preparation: The polymer pellets are ground in anultracentrifugal mill (Retsch ZM 100) with a sieve with 2 mm holes. Thepellets are cooled down with liquid nitrogen. 5 g of the ground polymeris extracted in 50 ml of cyclohexane at a temperature of 81° C. for 2hours. If needed, cyclohexane is then added to exact 50 ml again. Thesolution is cooled down in room temperature and thereafter the polymeris precipitated with 50 ml iso-propanol. A suitable amount of thesolution is filtered and injected into HPLC equipment.

The HPLC measurement can e.g. be performed with a reversed phase C-18column and methanol and water as mobile phase, for example in a ratio of85:15. A UV detector can be used, wavelength 230 nm for Irganox 1010,Irganox 1330 and Irgafos 168. The quantification is made usingcalibration curves in a conventional manner.

The following parameters further define the method used:

Instrument: Agilent 1200

Column: Zorbax C18-SB (150×4.6 mm)

Column temperature: 40° C.

Flow: 1 ml/min

Injection volume: 10 μl

Eluent: methanol/water

Gradient:0 min 85% methanol/15% water, 6 min 100% methanol, 23 min 100%methanol

Detection: UV wavelength 230 nm

Retention times:

Irganox 1010 10.0 min Irganox 1330 10.7 min Irgafos 168 18.8 minPhosphate 12.6 min

Phosphate is obtained by hydrolysis from Irgafos168.

2. Lifetime of Pipes Comprising Different Antioxidants

The compositions for the examples were compounded/melt homogenized in aBuss-Co-Kneader 100 MDK/E-11 L/D. Polymer and additives were fed intothe first mixer inlet of the Buss Co-Kneader which is a single screwextruder with a downstream discharge single extruder with pelletizingunit cutting pellets in molten stage and cooled via water. The mixertemperature profile was 91/164/193/189/196° C. from first inlet tooutlet, and discharge extruder temperature 113° C. The mixer screw rpmwas 195 rpm and the throughput 175 kg/h.

Pipes 12 mm×2 mm (outer diameter×wall thickness) were prepared byextrusion in a Battenfeld 45-25B extruder, which gave an output of 15kg/h at a screw speed of 20 rpm. The extruder melt temperature was 218°C.

Polyolefin (A) used as base resin in all examples is an unstabilisedbimodal medium density polyethylene with a total 1-butene comonomercontent of 3.1 wt % whereby the comonomer is present exclusively in thehigh molecular weight part of the polyethylene, an MFR₅ of 0.81 g/10min, and a density of 941 kg/m³.

The additives which were added to the base resins to yield thepolyethylene compositions used for pipe production are given in Table 1.If not indicated otherwise, the values are given in wt %. Still further,in Table 1 also the results of the lifetime tests in ClO₂-containingwater are given.

A carbon black master batch was used containing 60.4 wt % high densitypolyethylene, 39.5 wt % carbon black and 0.1 wt % Irganox 1010.

In the Examples according to the invention (Examples 1 and 2) a mixtureof two antioxidants is used, namely1,3,5-Trimethyl-2,4,6-tris-(3,5-di-tert. butyl-4-hydroxy-phenyl) benzene(CAS No. 1709-70-2, Irganox 1330 from Ciba Speciality Chemicals) andTris(2,4-di-tert. butylphenyl)phosphite (CAS No. 31570-04-4, Irgafos 168from Ciba Speciality Chemicals).

In Comparative Examples 1 and 2 a typical conventional mixture ofantioxidants is used, namely Tris(2,4-di-tert. butylphenyl)phosphite(CAS No. 31570-04-4, Irgafos 168 from Ciba Speciality Chemicals) andPentaerythrityl-tetrakis(3-(3′,5′-di-tert.butyl-4-hydroxyphenyl)propionate) (CAS No. 6683-19-8, Irganox 1010, fromCiba Speciality Chemicals).

Hence, in the inventive Examples Irganox 1010 used in the ComparativeExamples is substituted by Irganox 1330. It can be seen that theinventive combination of antioxidants achieves a highly improvedresistance against ClO₂-containing water.

TABLE 1 Comp. Comp. Example 1 Example 2 Example 1 Example 2 base resin93.88 93.55 93.88 93.55 calcium stearate 0.15 0.15 0.15 0.15 carbonblack MB 5.75 5.75 5.75 5.75 Irganox 1330/ppm 1100 4400 — — Irganox1010/ppm — — 1100 4400 Irgafos 168/ppm 1100 1100 1100 1100 antioxidants,total/ppm 2200 5500 2200 5500 ClO₂-resistance 1/h^(a) 2839 3827 13982135 ClO₂-resistance 2/h^(a) 2594 4143 1670 2833 average value/h 27173985 1534 2484 ^(a)until failure

1. A method for increasing the lifetime of a pipe or a fitting made of apolyolefin composition, produced in a sequential multistage processutilizing reactors coupled in series, comprising using a combination oftwo antioxidants (B) and (C) in said polyolefin composition, whereinsaid antioxidants are the following: antioxidant (B) with generalformula R¹—P(OAr)₂, wherein OAr is according to formula (I):

wherein R¹ is a non-substituted or substituted aliphatic or aromatichydrocarbyl radicals which may comprise heteroatoms, R², R³, R⁴, R⁵ andR⁶ independently are a hydrogen atom or non-substituted or substitutedaliphatic or aromatic hydrocarbyl radicals which may compriseheteroatoms, and antioxidant (C) according to formula (II):

wherein R⁷, R⁸ and R⁹ independently are non-substituted or substitutedaliphatic or aromatic hydrocarbyl radicals which may comprise OH-groups,X¹, X², and X³ independently are H or OH, with the proviso that at leastone of X¹, X² and X³ is OH, and the entire molecule does not comprise anester group; wherein the pipe or fitting is in permanent contact withchlorine dioxide-containing water.
 2. A method according to claim 1,wherein the polyolefin composition further comprises a base resin (A),which comprises an ethylene homo- or copolymer.
 3. The method accordingto claim 2, wherein the base resin (A) of the polyolefin compositionconsists of an ethylene homo- or copolymer.
 4. The method according toclaim 1, wherein R¹ is OAr.
 5. The method according to claim 1, whereinR³ and R⁵ are hydrogen atoms and at least one of R², R⁴ and R⁶ is analkyl radical.
 6. The method according to claim 1, wherein R⁷ and R⁸ arealkyl radicals with 3 to 10 carbon atoms.
 7. The method according toclaim 1, wherein R⁹ has from 30 to 70 carbon atoms and includes one ormore phenyl residues.
 8. The method according to claim 1, wherein thesum of concentration of antioxidants (B) and (C) is between 1500 and6000 ppm.
 9. The method according to claim 1, wherein said increasedlifetime of the pipe or fitting is shown in a hoop stress test accordingto ASTM F 2263-03 compared to a pipe or fitting made of a correspondingpolyolefin composition comprising antioxidants or a combination ofantioxidants other than the combination of antioxidants (B) and (C) inan equal concentration.
 10. The method according to claim 9, whereinsaid increased lifetime of the pipe or fitting is shown by a failuretime of at least 2500 hours in a hoop stress test according to ASTM F2263-03.